CISC 3320 File System Implementation - GitHub Pages

CISC 3320

File System

ImplementationHui Chen

Department of Computer & Information Science

CUNY Brooklyn College

12/2/2019 1CUNY | Brooklyn College

Acknowledgement

• These slides are a revision of the slides

provided by the authors of the textbook

via the publisher of the textbook

12/2/2019 CUNY | Brooklyn College 2

Outline

• File-System Structure

• File-System Operations

• Directory Implementation

• Allocation Methods

• Free-Space Management

• Efficiency and Performance

• Recovery

• Example: WAFL File System


File-System Structure

• File structure

• Logical storage unit

• Collection of related information

• File system resides on secondary storage (disks)

• Provided user interface to storage, mapping logical to physical

• Provides efficient and convenient access to disk by allowing data to be stored, located retrieved easily

• Disk provides in-place rewrite and random access

• I/O transfers performed in blocks of sectors (usually 512 bytes)

• File control block (FCB) – storage structure consisting of information about a file

• Device driver controls the physical device

• File system organized into layers


File-System

Layers


File System Layers: Device

Drivers• Device drivers manage I/O devices at

the I/O control layer

• Outputs low-level hardware specific

commands to hardware controller

• Example:

• Given commands like “read drive1, cylinder 72,

track 2, sector 10, into memory location 1060”

(CHS addressing) or “read drive1, block 1442, into

memory location 1060” (LBA)


File System Layers: Basic File

System• Basic file system

• Given command like “retrieve block 123”

translates to device driver

• Also manages memory buffers and

caches (allocation, freeing, replacement)

• Buffers hold data in transit

• Caches hold frequently used data


File System Layers: File

Organization Module• File organization module understands

files, logical address, and physical blocks

• Translates logical block # to physical block #

• Manages free space, disk allocation


File System Layers: Logical

File System• Logical file system manages metadata

information

• Translates file name into file number, file

handle, location by maintaining file control

blocks (inodes in UNIX)

• Directory management

• Protection


File System Layers: Benefits

and Overhead• Layering useful for reducing complexity

and redundancy

• Duplication of code is minimized.

• The I/O control and sometimes the basic file-

system code can be used by multiple file systems.

• Each file system can then have its own logical file-

system and file-organization modules.

• But layering adds overhead and can

decrease performance


File Systems: Examples

• Many file systems, sometimes many within an operating system

• Each with its own format, e.g.,

• CD-ROM is ISO 9660;

• Unix has UFS, FFS;

• Windows has FAT, FAT32, NTFS as well as floppy, CD, DVD Blu-ray;Linux has more than 130 types, with extended file system ext3 and ext4 leading; plus distributed file systems, etc.)

• New ones still arriving – ZFS, GoogleFS, Oracle ASM, FUSE


Questions?

• File system structure and layering


File-System Operations

• On-disk and in-memory structures

• We have system calls at the API level, but how do we implement their functions?

• Boot control block contains info needed by system to boot OS from that volume

• Needed if volume contains OS, usually first block of volume

• Volume control block (superblock, master file table) contains volume details

• Total # of blocks, # of free blocks, block size, free block pointers or array

• Directory structure organizes the files

• Names and inode numbers, master file table12/2/2019 CUNY | Brooklyn College 13

File-System Operations:

Control Blocks• Boot control block contains info needed by system to boot OS from that volume

• Needed if volume contains OS, usually first block of volume

• Volume control block (superblock, master file table) contains volume details

• Total # of blocks, # of free blocks, block size, free block pointers or array


File-System Operations:

Directory Structures• Directory structure organizes the files

• Names and File Control Block (e.g., inode)

numbers, master file table


File-System Operations: Per-

File File Control Block• Per-file File Control Block (FCB)

contains many details about the file

• typically inode number, permissions, size,

dates

• NFTS stores into in master file table using

relational DB structures


In-Memory File System

Structures• Mount table storing file system mounts, mount

points, file system types

• system-wide open-file table contains a copy of the FCB of each file and other info

• per-process open-file table contains pointers to appropriate entries in system-wide open-file table as well as other info

• Examples: (a) opening file;(b) reading a file

• buffers hold data blocks from secondary storage

• Open returns a file handle for subsequent use

• Data from read eventually copied to specified user process memory address


Directory Implementation

• Linear list of file names with pointer to the data blocks

• Simple to program

• Time-consuming to execute

• Linear search time

• Could keep ordered alphabetically via linked list or use B+ tree

• Hash Table – linear list with hash data structure

• Decreases directory search time

• Collisions – situations where two file names hash to the same location

• Only good if entries are fixed size, or use chained-overflow method


Questions?

• File system operations and supporting

data structures

• On-disk structures

• In-memory structures

• Directory implementation


Allocation Methods

• Continuous allocation

• Linked allocation

• Indexed allocation

• Combined scheme


Allocation Methods:

Contiguous• An allocation method refers to how disk blocks

are allocated for files:

• Contiguous allocation – each file occupies set of contiguous blocks

• Best performance in most cases

• Simple

• only starting location (block #) and length (number of blocks) are required

• Problems include

• finding space for file, knowing file size, external fragmentation, need for compaction off-line (downtime) or on-line



• Mapping from logical to physical

LA/512

Q

R

Block to be accessed = Q +

starting address

Displacement into block = R

Extent-Based Systems

• A modified contiguous allocation scheme

used in many newer file systems (e.g.,

Veritas File System)

• Extent-based file systems allocate disk

blocks in extents

• An extent is a contiguous block of disks

• Extents are allocated for file allocation

• A file consists of one or more extents


Allocation Methods - Linked

• Linked allocation – each file a linked list of blocks

• File ends at nil pointer

• No external fragmentation

• Each block contains pointer to next block

• No compaction, external fragmentation

• Free space management system called when new block needed

• Improve efficiency by clustering blocks into groups but increases internal fragmentation

• Reliability can be a problem

• Locating a block can take many I/Os and disk seeks


Linked Allocation

• Each file is a linked list of disk blocks: blocks

may be scattered anywhere on the disk

• Mapping

• Block to be accessed is the Qth block in the

linked chain of blocks representing the file.

• Displacement into block = R + 1


pointerblock =

LA/511

Q

R

FAT Variation

• FAT (File Allocation Table) variation

• Much like a linked list, but faster on disk and

cacheable

• Beginning of volume has table, indexed by

block number

• New block allocation simple


File-Allocation Table


Allocation Methods - Indexed


• Each file has its own index block(s) of

pointers to its data blocks

• Logical view


index table

Indexed Allocation

• Need index table

• Random access

• Dynamic access without external fragmentation, but have overhead of index block

• Mapping from logical to physical in a file of maximum size of 256K bytes and block size of 512 bytes. We need only 1 block for index table


LA/512

Q

R

Q = displacement into index tableR = displacement into block

Mapping in Indexed Allocation

• Mapping from logical to physical in a file of unbounded length (block size of 512 words)

• Linked scheme – Link blocks of index table (no limit on size)


LA / (512 x 511)

Q1

R1

Q1 = block of index table

R1 is used as follows:

R1 / 512

Q2

R2

Q2 = displacement into block of index table

R2 displacement into block of file:

• Two-level index (4K blocks could store

1,024 four-byte pointers in outer index

→ 1,048,567 data blocks and file size of

up to 4GB)



LA / (512 x 512)

Q1

R1

Q1 = displacement into outer-index

R1 is used as follows:

R1 / 512

Q2

R2

Q2 = displacement into block of index table

R2 displacement into block of file:

Combined Scheme: UNIX

UFS


More index blocks than can be addressed with 32-bit file pointer

4K bytes per block, 32-bit addresses

Performance• Best method depends on file access type

• Contiguous great for sequential and random

• Linked good for sequential, not random

• Declare access type at creation -> select either contiguous or linked

• Indexed more complex

• Single block access could require 2 index block reads then data block read

• Clustering can help improve throughput, reduce CPU overhead

• For NVM, no disk head so different algorithms and optimizations needed

• Using old algorithm uses many CPU cycles trying to avoid non-existent head movement

• With NVM goal is to reduce CPU cycles and overall path needed for I/O


Performance

• Adding instructions to the execution path to save one disk I/O is reasonable

• Intel Core i7 Extreme Edition 990x (2011) at 3.46Ghz = 159,000 MIPS

• http://en.wikipedia.org/wiki/Instructions_per_second

• Typical disk drive at 250 I/Os per second

• 159,000 MIPS / 250 = 630 million instructions during one disk I/O

• Fast SSD drives provide 60,000 IOPS

• 159,000 MIPS / 60,000 = 2.65 millions instructions during one disk I/O


Questions?

• Continuous allocation

• Linked allocation


• Combined scheme


Free-Space Management

• File system maintains free-space list to

track available blocks/clusters

• (Using term “block” for simplicity)


Bit Vector/Map

• (n blocks)


…

0 1 2 n-1

bit[i] = 1 block[i] free

0 block[i] occupied

Block number calculation

(number of bits per word) *

(number of 0-value words) +

offset of first 1 bit

CPUs have instructions to return offset within word of first “1” bit

• Bit map requires extra space

• Example:

block size = 4KB = 212 bytes

disk size = 240 bytes (1 terabyte)

n = 240/212 = 228 bits (or 32MB)

if clusters of 4 blocks -> 8MB of memory

• Easy to get contiguous files


Linked Free Space List on

Disk


• Linked list (free list)

• Cannot get contiguous

space easily

• No waste of space

• No need to traverse the

entire list (if # free blocks

recorded)

Grouping and Counting

• Grouping • Modify linked list to store address of next n-1

free blocks in first free block, plus a pointer to next block that contains free-block-pointers (like this one)

• Counting• Because space is frequently contiguously used

and freed, with contiguous-allocation allocation, extents, or clustering

• Keep address of first free block and count of following free blocks

• Free space list then has entries containing addresses and counts


Space Maps• Used in ZFS

• Consider meta-data I/O on very large file systems

• Full data structures like bit maps couldn’t fit in memory -> thousands of I/Os

• Divides device space into metaslab units and manages metaslabs

• Given volume can contain hundreds of metaslabs

• Each metaslab has associated space map

• Uses counting algorithm

• But records to log file rather than file system

• Log of all block activity, in time order, in counting format

• Metaslab activity -> load space map into memory in balanced-tree structure, indexed by offset

• Replay log into that structure

• Combine contiguous free blocks into single entry12/2/2019 CUNY | Brooklyn College 46

TRIMing Unused Blocks

• HDDS overwrite in place so need only free list

• Blocks not treated specially when freed

• Keeps its data but without any file pointers to it, until overwritten

• Storage devices not allowing overwrite (like NVM) suffer badly with same algorithm

• Must be erased before written, erases made in large chunks (blocks, composed of pages) and are slow

• TRIM is a newer mechanism for the file system to inform the NVM storage device that a page is free

• Can be garbage collected or if block is free, now block can be erased


Efficiency

• Efficiency dependent on:

• Disk allocation and directory algorithms

• Types of data kept in file’s directory entry

• Pre-allocation or as-needed allocation of

metadata structures

• Fixed-size or varying-size data structures


Performance

• Performance

• Keeping data and metadata close together

• Buffer cache – separate section of main memory for frequently used blocks

• Synchronous writes sometimes requested by apps or needed by OS

• No buffering / caching – writes must hit disk before acknowledgement

• Asynchronous writes more common, buffer-able, faster

• Free-behind and read-ahead – techniques to optimize sequential access

• Reads frequently slower than writes


Page Cache

• A page cache caches pages rather than

disk blocks using virtual memory

techniques and addresses

• Memory-mapped I/O uses a page cache

• Routine I/O through the file system uses

the buffer (disk) cache


I/O Without a Unified Buffer

Cache


Unified Buffer Cache

• A unified buffer cache uses the same

page cache to cache both memory-

mapped pages and ordinary file system

I/O to avoid double caching

• But which caches get priority, and what

replacement algorithms to use?


I/O Using a Unified Buffer

Cache


Questions?

• Efficiency

• Performance

• Cache


Recovery

• Consistency checking – compares data in

directory structure with data blocks on disk,

and tries to fix inconsistencies

• Can be slow and sometimes fails

• Use system programs to back up data from

disk to another storage device (magnetic

tape, other magnetic disk, optical)

• Recover lost file or disk by restoring data

from backup


Log Structured File Systems

• Log structured (or journaling) file systems record each metadata update to the file system as a transaction

• All transactions are written to a log

• A transaction is considered committed once it is written to the log (sequentially)

• Sometimes to a separate device or section of disk

• However, the file system may not yet be updated

• The transactions in the log are asynchronously written to the file system structures

• When the file system structures are modified, the transaction is removed from the log

• If the file system crashes, all remaining transactions in the log must still be performed

• Faster recovery from crash, removes chance of inconsistency of metadata


Example: WAFL File System

• Used on Network Appliance “Filers” –distributed file system appliances

• “Write-anywhere file layout”

• Serves up NFS, CIFS, http, ftp

• Random I/O optimized, write optimized

• NVRAM for write caching

• Similar to Berkeley Fast File System, with extensive modifications


The WAFL File Layout


Snapshots in

WAFL


Questions?

• Recovery

• Log structured file systems

• WAFL


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